Single stage cleaning of metal surfaces with aqueous alkaline cleaning solutions is known. The cleaning solutions remove coolants and lubricants employed in machining operations. Metal fines and other contaminants from metal forming and machining operations are also removed by the cleaning process. Conventional cleaners frequently result in a surface finish which is susceptible to oxidation and/or an increased coefficient of friction over time. For metal articles which are machined to close tolerances such surface degradation can have a deleterious effect on later operations such as automated assembly operations. In assembly operations by automated equipment such surface degradation i.e., high coefficient of friction, may cause jamming or require decreased operating speeds for the equipment. High coefficients of friction may also cause automated assembly equipment to reject an excessive number of parts. An excessively thick oxide layer may actually prevent entry of a part into an automated machining process.
The use of automated machinery and assembly equipment is common, for example in the production of automotive engines and transmissions. A need exists in the metal treatment industry for cost effective, simple means to modify the coefficient of friction of machined articles and inhibit oxide growth in order to improve their assembly properties. A reduction in the coefficient of friction and the maintenance of a low coefficient of friction over time will improve the ease of assembly for articles machined to close mechanical tolerances.
Those practiced in the art know that the coefficient of static friction between two surfaces is almost always larger than the coefficient of kinetic friction. A high coefficient of static friction is generally a limiting factor in assembly operation speed. A reduction in the coefficient of static friction will improve, by decreasing, the rejection rate of parts by automatic assembly equipment. This will allow more efficient production. It also may be possible to increase the speed of the assembly operation.
It is therefore desirable to improve, by decreasing, the coefficient of friction of machined metal articles which are cleaned by an aqueous alkaline cleaner. It is an object of the present invention to improve the coefficient of friction of machined metal articles which are cleaned by aqueous alkaline cleaners.
Lubricity-imparting additives are known in the aluminum beverage container industry. See for example U.S. Pat. Nos. 4,859,351 Awad and 5,061,389, Reichgott. The additives described in the prior art are preferably applied after cleaning and rinsing of the aluminum. The additives described in the prior art may be inappropriate in single-stage cleaning solutions for several reasons. The prior art additives, such as ethoxylated fatty acids and polyethylene glycol esters may not resist alkaline hydrolysis in an alkaline cleaner concentrate or cleaner bath. The delay between cleaning and subsequent machining steps in the present invention may be several weeks where as the delays between cleaning and printing steps for beverage containers is typically on the order of minutes.
The requirements of a single stage cleaning process are substantially different than other metal cleaning operations. Only a single pH may be used. Intermediate rinses are not available to remove soils and metallic fines. The cleaning solution must drain efficiently from the cleaned surface. Also, the growth of oxide layers on the metal surface must be controlled after oils have been removed by the cleaner. The current use of chlorinated solvents to accomplish these goals is under increasing pressure from both health and environmental regulation.